Field of the Invention
The present invention relates to a focus-detection device and a method for controlling the same, and relates particularly to a focus-detection device that performs phase-difference detection type focus detection based on outputs of an image sensor, and a method for controlling the same. The present invention also relates to an image capture apparatus comprising a focus-detection device.
Description of the Related Art
Japanese Patent Laid-Open No. 2008-52009 discloses an apparatus that perform pupil-division type focus detection using an image sensor in which each of two-dimensionally arranged pixels has a microlens. In this apparatus, a photoelectric conversion portion of each pixel has two divided areas that receive, via the microlens, light beams having passed through different regions of an exit pupil of a photographic lens. A correlation operation can be performed on a pair of output signals generated by the plurality of pixels for the respective divided areas so as to compute a phase difference (shift amount) between the pair of output signals, and a defocus amount can be computed based on the phase difference.
Furthermore, Japanese Patent Laid-Open No. 2008-52009 (see FIGS. 24 and 25) and Japanese Patent No. 3592147 disclose image sensors in which some pixels serve as focus detection pixels for use in pupil-division type focus detection. Although outputs of the focus detection pixels need to be corrected in order to be used as outputs of imaging pixels, the number of signals to be read as focus detection signals is smaller than that in the configuration in which a photoelectric conversion portion of each pixel is divided, making it possible to suppress the manufacturing costs of the image sensors and their computation costs in signal processing.
In the configurations using focus detection pixels, a pair of photoelectric conversion portions that receive via the microlenses light beams having passed through different regions of an exit pupil of a photographic lens are arranged in different pixels. That is, in a pair of output signals (an “A image” and a “B image”) for use in detecting a phase difference, the position of a pixel group for use in generating the A image and the position of a pixel group for use in generating the B image are different. Accordingly, the A image and the B image may have a low degree of similarity depending on a pattern of a subject image, and in such a case, the accuracy in focus detection may deteriorate.
Furthermore, if the focus detection pixels are disposed at a large distance, there may be the case where frequency components of a high-frequency band of a subject optical image cannot be acquired. Therefore, aliasing occurs differently between the A image and the B image, causing an error in focus detection.